1. Field of the Invention
The present invention relates to an optical head for an information processing apparatus for optically recording information on an information recording medium and optically retrieving information from the information recording medium and, more particularly, to an improvement in an optical means for splitting an optical path in the optical head.
2. Description of the Related Art
An optical information recording/reproduction apparatus, e.g., an optical filing apparatus is assembled with an optical head for recording information on an optical information recording medium, i.e., an optical disk, and retrieving information therefrom. As an optical head of this type, an erasable opto-magnetic optical head, a write once type optical head, and the like are known. In the opto-magnetic optical head, recorded information is retrieved by a light beam radiated onto a recording surface of an optical disk by utilizing an opto-magnetic effect, and information is recorded and erased by the light beam. In the write once type optical head, a light beam is radiated on a recording surface of an optical disk, and recorded information is retrieved by modulating a light intensity of the reflected light beam or transmission light beam.
The optical head described above comprises a converging mean for converging a light beam emitted from a semiconductor laser as a light source on an information recording surface of an optical disk, a light beam splitting means for directing the light beam reflected by the optical disk toward a photosensor as a signal detection means, and a photosensor and a signal processing means for detecting the split beam and generating a retrieval signal of information, a focusing control signal, and a tracking control signal.
In the opto-magnetic optical head, a light beam emitted from a semiconductor laser is radiated onto an optical disk through a splitting means, i.e., a polarization beam split surface formed on a polarization prism, and the light beam reflected by the optical disk is reflected by the polarization beam split surface of the prism to be guided to a photosensor through a 1/2 wavelength plate. The 1/2 wavelength plate is arranged to define an angle of 22.5.degree. between an axial direction of an ordinary or extraordinary ray passing through the 1/2 wavelength plate and a prism bottom surface. In an optical system in which the 1/2 wavelength plate is arranged to satisfy such a relationship, a rotary adjustment mechanism for rotating and adjusting the optical axis of the 1/2 wavelength plate around the system optical axis to assure stability of a retrieval signal is normally arranged in the opto-magnetic optical head.
In the write once type optical head, a light beam emitted from a laser beam is radiated on an optical disk through a polarization beam splitter formed on a prism, and a 1/4 wavelength plate, and the light beam reflected by the optical disk passes through the 1/4 wavelength plate again. The light beam is then reflected by a polarization beam split surface formed on the prism, and is then guided toward a photosensor.
In the above-mentioned optical heads, the 1/2 or 1/4 wavelength plate is arranged, and the polarization beam split surface is formed on the prism to split a light beam reflected by the optical disk and to guide it toward the photosensor. The 1/2 and 1/4 wavelength plates are expensive, and an optical system assembled with the 1/2 or 1/4 wavelength plate becomes large in size.
In the opto-magnetic optical head, the 1/2 wavelength plate must be arranged to satisfy the predetermined positional relationship described above, and the adjustment mechanism therefor is arranged. When the 1/2 wavelength plate does not satisfy the predetermined positional relationship, the light intensities of P and S components detected by two detectors are unbalanced. In addition, an optical system becomes bulky due to the presence of the adjustment mechanism. When the 1/2 wavelength plate is fixed to the prism to attain a compact structure, its adjustment is cumbersome. If adjustment is unsuccessful, detected P and S components are unbalanced, and an S/N ratio of a retrieval signal may often be impaired. The unbalanced P and S components can be corrected by a correction circuit connected to the output side of the detectors. However, when a large correction amount is to be set, stability of the correction circuit is impaired.
In the write once type optical head, the polarization beam split split surface has a characteristic of allowing a specific polarized light component, e.g., a P component to pass through. When a semiconductor laser which generates a linearly polarized laser beam is arranged at a position rotated from a predetermined position about its optical axis, a plane of polarization of a laser beam incident on the polarization beam split surface is rotated, and a light intensity of a laser beam is changed. As a result, characteristics of optical heads of even the same type may vary. Mounting and adjustment operations of the semiconductor laser, and associated adjustments are cumbersome, resulting in a poor manufacturing yield of optical heads.